Evolution may lead to speciation

Individuals in a population look and function differently from one another — this is phenotypic variation. Genes and the environment both cause it, but mutation is the original source of all genetic differences.

Genetics, populations, evolution and ecosystems

Evolution may lead to speciation

Sexual reproduction shuffles genetic information in two ways. Meiosis creates genetically unique sex cells, and random fertilisation combines them unpredictably, producing new gene combinations every time.

Genetics, populations, evolution and ecosystems

Evolution may lead to speciation

Not every organism survives long enough to reproduce. Predators, disease, and competition for resources mean that individuals with helpful traits survive and breed more than others.

Genetics, populations, evolution and ecosystems

Evolution may lead to speciation

Organisms whose traits help them survive in their environment tend to reproduce more. They pass the alleles — gene variants — behind those helpful traits to their offspring.

Genetics, populations, evolution and ecosystems

Evolution may lead to speciation

Some individuals reproduce more than others because their traits suit the environment better. Over generations, the alleles — gene variants — behind those traits become more common in the population.

Genetics, populations, evolution and ecosystems

Evolution may lead to speciation

Natural selection can change a population's traits in three ways. It can keep traits the same, shift them toward one extreme, or split the population into two distinct groups.

Genetics, populations, evolution and ecosystems

Evolution may lead to speciation

Evolution means the allele frequencies in a population shift over generations. An allele is one version of a gene, and its frequency is how common it is in the population.

Genetics, populations, evolution and ecosystems

Evolution may lead to speciation

When two populations stop interbreeding, their alleles change independently over time. Mutations and natural selection build up separately in each group, making the two gene pools increasingly different.

Genetics, populations, evolution and ecosystems

Evolution may lead to speciation

A species is defined by the biological species concept: members of the same species can interbreed and produce fertile offspring. Speciation — the formation of a new species — occurs when this ability breaks down permanently. The process typically follows these steps: 1. A single population splits into two groups that are reproductively isolated (prevented from interbreeding), for example by a p

Genetics, populations, evolution and ecosystems

Evolution may lead to speciation

Speciation is the formation of new species. It can happen when a physical barrier splits a population apart (allopatric), or when reproductive isolation develops within the same location (sympatric).

Genetics, populations, evolution and ecosystems

Evolution may lead to speciation

Genetic drift is random chance changing how common an allele is in a population. It has the biggest effect when a population is small.

Genetics, populations, evolution and ecosystems

Evolution may lead to speciation

Natural selection acts on the genetic variation present in a population, meaning individuals with phenotypes — observable characteristics — that suit their environment are more likely to survive, reproduce, and pass on their alleles (the variants of a gene responsible for those traits). Over generations, this shifts allele frequencies within the gene pool (the total collection of alleles in a population), which is what evolution actually is at a genetic level. When populations become reproductively isolated — separated so that they can no longer interbreed — their gene pools diverge independently until members can no longer produce fertile offspring together, giving rise to new species through a process called speciation.

Genetics, populations, evolution and ecosystems

Inheritance

The genotype is the complete set of alleles an organism carries in its DNA. Think of it as the genetic instruction manual — written in the cells, not always visible from the outside.

Genetics, populations, evolution and ecosystems

Inheritance

Your phenotype is every observable characteristic you have — like eye colour or height. It comes from your genes working together with your environment.

Genetics, populations, evolution and ecosystems

Inheritance

A single gene can exist in more than two versions, called alleles. Human blood group is a classic example — one gene has three different alleles.

Genetics, populations, evolution and ecosystems

Inheritance

Alleles are different versions of a gene. A dominant allele controls the phenotype even when only one copy is present. A recessive allele only shows its effect when both copies match. Codominant alleles both contribute to the phenotype together.

Genetics, populations, evolution and ecosystems

Inheritance

Diploid organisms carry two copies of every gene. When both copies are the same allele, the organism is homozygous. When the two copies differ, it is heterozygous.

Genetics, populations, evolution and ecosystems

Inheritance

A genetic diagram uses a Punnett square — a grid showing every possible combination of parental gametes — to predict offspring outcomes. Follow these steps for any cross: 1. Write the parental phenotypes and genotypes (e.g. Tall Tt × short tt). 2. Identify the gametes each parent can produce and place them along the grid axes. 3. Fill the grid to find all possible offspring genotypes. 4. Convert

Genetics, populations, evolution and ecosystems

Inheritance

Some genes break the simple rules of inheritance. They can sit on sex chromosomes, travel together on the same chromosome, exist in more than two versions, or have one gene switch another gene off entirely.

Genetics, populations, evolution and ecosystems

Inheritance

The chi-squared test is a statistical calculation. It tells you whether the difference between your actual breeding results and your predicted ratio is due to chance or something real.

Genetics, populations, evolution and ecosystems

Inheritance

Inheritance is the study of how alleles — the different versions of a gene — are passed from parents to offspring and how they determine an organism's characteristics. By constructing genetic diagrams, you can predict the phenotype (the observable traits an organism displays) that results from any combination of alleles, whether those alleles are dominant, recessive, codominant, sex-linked, or influenced by other genes through epistasis. Mastering these patterns is essential groundwork for understanding how variation arises within populations, which underpins everything covered in evolution and speciation later in this section.

Genetics, populations, evolution and ecosystems

Populations

A species can live as a single group in one place, or as several separate groups spread across different locations. Each of these groups is called a population.

Genetics, populations, evolution and ecosystems

Populations

A population has three defining features that you must be able to state precisely. 1. Same species — all individuals share enough genetic similarity to produce fertile offspring with one another. 2. Same space — they occupy a defined area, for example all the red deer living on the Isle of Rum in Scotland. 3. Same time — they exist together at the same moment, so you are not mixing individuals fr

Genetics, populations, evolution and ecosystems

Populations

A gene pool is every allele (version of a gene) that exists across all individuals in a population. Allele frequency measures how common each allele is within that pool.

Genetics, populations, evolution and ecosystems

Populations

The Hardy–Weinberg principle is a mathematical model. It predicts that, in a stable population, the proportion of each allele (version of a gene) stays the same across generations.

Genetics, populations, evolution and ecosystems

Populations

The Hardy–Weinberg principle only holds true when a population meets specific conditions. These conditions include no mutation, no natural selection, random mating, no migration, and a large population size.

Genetics, populations, evolution and ecosystems

Populations

Every gene in a population exists in different versions called alleles. The Hardy–Weinberg equation, p² + 2pq + q² = 1, describes the expected frequencies of all three possible genotypes when a gene has two alleles. Here p is the frequency of one allele (usually the dominant one) and q is the frequency of the other (usually the recessive one). Because these are the only two alleles, p + q = 1. T

Genetics, populations, evolution and ecosystems

Populations

Within a species, individuals live in populations — groups occupying the same area at the same time that can interbreed — and together these individuals share a gene pool, meaning the complete set of alleles (versions of genes) present across the group. The Hardy–Weinberg principle is a mathematical model that predicts allele frequencies will remain stable between generations, provided certain conditions are met, such as no mutation, no natural selection, and random mating. Using the Hardy–Weinberg equation, p² + 2pq + q² = 1, you can calculate the expected frequencies of alleles, genotypes, and phenotypes in a population — a skill that underpins your understanding of how and why populations change over time through evolution.

Genetics, populations, evolution and ecosystems

Populations in ecosystems

A community is all the different species living together in one place. An ecosystem includes that community plus all the non-living surroundings, such as soil, water, and temperature.

Genetics, populations, evolution and ecosystems

Populations in ecosystems

Every species fills a unique role in its habitat, called a niche. Adaptations to non-living conditions (like temperature) and living ones (like predators) define that role.

Genetics, populations, evolution and ecosystems

Populations in ecosystems

Carrying capacity is the maximum population size an ecosystem can sustain over time, given its available resources. A population typically grows rapidly when numbers are low, then levels off as it approaches carrying capacity — producing an S-shaped (logistic) growth curve. Three categories of factor cause carrying capacity to vary: 1. Abiotic factors — non-living conditions such as temperature,

Genetics, populations, evolution and ecosystems

Populations in ecosystems

A quadrat is a square frame placed on the ground to define a sample area. Researchers count every individual of the target species inside it, then repeat this across many randomly placed quadrats and calculate a mean density. Multiplying that mean by the total habitat area gives a population estimate. For species distributed unevenly across a habitat — for example, where bluebells grow more densel

Genetics, populations, evolution and ecosystems

Populations in ecosystems

The mark-release-recapture method only gives an accurate population estimate if certain conditions hold true. These conditions are the assumptions scientists must accept when using this technique.

Genetics, populations, evolution and ecosystems

Populations in ecosystems

Primary succession is the process where living things gradually colonise bare, lifeless ground. Over time, each wave of species changes the environment until a stable final community establishes itself.

Genetics, populations, evolution and ecosystems

Populations in ecosystems

As succession progresses, each group of species alters the environment around it. Those changes make conditions better for new species and worse for the ones already there.

Genetics, populations, evolution and ecosystems

Populations in ecosystems

As organisms grow and die, they physically alter their surroundings. These changes make the environment less harsh, allowing new species to survive there and increasing biodiversity.

Genetics, populations, evolution and ecosystems

Populations in ecosystems

Left alone, most habitats change over time until a dense climax community takes over. Conservationists actively intervene to hold a habitat at an earlier stage, protecting species that would otherwise disappear.

Genetics, populations, evolution and ecosystems

Populations in ecosystems

Every ecosystem — a community of interacting species together with the non-living environment they inhabit — has a carrying capacity, meaning a maximum population size it can sustainably support, shaped by factors such as competition, predation, and abiotic conditions like temperature and water availability. Each species occupies a niche, the unique role and set of conditions it is adapted to within that habitat, and understanding niches helps explain why populations fluctuate over time. This subtopic also covers how ecosystems change through primary succession, the gradual process by which pioneer species colonise bare ground and progressively alter conditions until a stable climax community is established, and how scientists use field techniques such as quadrats and mark-release-recapture to estimate population sizes reliably.

Genetics, populations, evolution and ecosystems